Hollow concrete elements are used in various constructions, often as buried pipes, but also as constructional elements in buildings, bridges, towers, etc.
Elongated reinforced concrete structures are used in a variety of fields. Examples of elongated reinforced concrete structures include masts and towers, pylons, chimneys, architectural structures, straight and curved beams, etc.
Traditionally, such elongated structures are cast in moulds onsite, either in one single moulding or in a sequence of moulding steps wherein reinforcement elements moulded in a preceding moulding are integrated in the subsequent moulding to achieve a continuous or overlapping longitudinal reinforcement structure throughout the structure. However, onsite moulding is time- and labour-consuming, as well as requiring the transportation of moulding equipment to the site. Moreover it is difficult onsite to completely control all the parameters of the moulding process which means that the material properties of the finished structure are likely to be suboptimal. As a direct consequence of high risk that the final structure will suffer from suboptimal material properties, to ensure an adequate safety margin, the structures must be overdimensioned.
An alternative to onsite moulding is the prefabrication of elongated concrete segments that can be assembled onsite. As the prefabrication of segments can be performed under well-controlled conditions and the whole segment can be moulded in one integral moulding, many of the above disadvantages are avoided.
One method for making elongated concrete structures, such as pipes, known as the “Hume” method and described, for example, in U.S. Pat. No. 3,577,505 involves rotating a drum mould which has end flanges which extend towards the longitudinal axis of the mould by a placing it on a drive roller and a driven roller. In modern devices the driving force of a motor is transmitted to the drive roller to rotate said drum mould such that an acceleration of centrifugal force of normally 3 to 5 g or even up to 15 g is obtained at an inner periphery of the mould.
In this condition, concrete is cast or poured into the drum mould by means of a movable conveyor as the conveyor is inserted longitudinally into the mould through an opening formed in one of the edge flanges. The deposited concrete is intended to form a concrete layer of uniform thickness in the inner surface of the drum mould and is prevented from leaving the drum mould by the edge flanges. The motor output is raised to increase the rotation of the drum mould to such an extent that acceleration of the centrifugal force in the proximity of the inner surface of the drum mould is raised to a range of 20 to 50 g. As a result, water is squeezed out of the concrete layer to tighten or physically harden said concrete layer. The time required for this step depends on the thickness of the finished pipe, the consistency of concrete and the magnitude of the centrifugal force. In general, however, about 15 minutes are needed for the pipe of 500 m/m in diameter while 40 to 60 minutes are needed for that of 3 meters in diameter. These devices rely on centrifugal force to distribute the concrete, remove excess water from it and compact it. This leads to long process times. This is a wet casting system and the concrete, even if some water is lost during the process, is still very wet when it is being cured. This means that the concrete is liable to suffer from shrinkage cracks during curing and due to the high centrifugal force applied during the curing the concrete also will become heterogeneous. Hence, this method is only really suitable for underground pipes which are subjected to low stress.
Another relatively successful method of manufacturing open-ended hollow concrete elements in the form of concrete pipe sections is the roller suspension method—known as the “Rocla” method. This method involves suspending a pipe mould which has end flanges which extend towards the central longitudinal axis of the mould, on a rotatable compaction roller which is aligned parallel to the pipe axis. As the compaction roller rotates, the mould, which is arranged about and suspended by its end flanges on the compaction roller, rotates about the compaction roller. Earth dry concrete is fed into the interior of the mould as the mould rotates and, since the mould is suspended on the compaction roller, the concrete is compacted in the nip between the inner surface of the mould and the outer surface of the compaction roller resulting in a well-compacted concrete and a relatively smooth pipe of uniform thickness. This roller suspension method of pipe formation is well-known and therefore is not described herein in any greater detail. See for example publication WO9836886 A1 and GB1391763. However this method suffers from the disadvantage that compaction of the concrete by compression in the nip between the compaction roller and the interior of the mould only takes place once the thickness of the concrete in the mould is sufficiently deep such that it comes into contact with the compaction roller. This leads to long process and compaction times. Additionally the contact pressure between the compaction roller and the concrete can never be higher than the weight of the mould plus concrete distributed on the contact area. The contact pressure depends on the geometry of the pipe and the amount of concrete actually present in the pipe, and producing consistent products is dependent on the skill of the operator of the machine.
Patent documents FR2872843, EP1645701 and DE2939472 describe segmented elongated concrete structures in the form of towers for wind turbines, but they fail to describe efficient ways of producing such elements. PCTSE2007/050306 discloses a segmented tower structure and a method for producing such elements and a method for producing such towers.
Some of the problems with existing solutions and methods are that they are inefficient and require high cement and chemical content ratios or produce a lot of waste during casting or have long curing times and/or suffer from shrinkage and the subsequent formation of cracks.